The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e

commit 5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e
Author: Scott Little <sclittle@chromium.org>
Date: Wed Jun 20 20:59:02 2018

LazyLoad: Record histograms around iframe deferral logic.

This CL makes Blink record histograms to measure how often LazyLoad
decides to immediately load a cross-origin iframe instead of deferring
it, or if the iframe is deferred, how often that iframe is later loaded
or seen by the user.

This will help with tuning the distance-from-viewport threshold for
determining how far down the page a lazy loaded frame should start
loading.

Since different distance-from-viewport thresholds will be used for
different effective connection types, separate counts for these
histograms are recorded for different effective connection types.

Bug:  854040 
Change-Id: Ied135413bb16770aeb22a304c312b8cc01df88b4
Reviewed-on: https://chromium-review.googlesource.com/1105606
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569008}
[modify] https://crrev.com/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e/third_party/blink/renderer/core/html/lazy_load_frame_observer.cc
[modify] https://crrev.com/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e/third_party/blink/renderer/core/html/lazy_load_frame_observer.h
[modify] https://crrev.com/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e/third_party/blink/renderer/core/html/lazy_load_frame_observer_test.cc
[modify] https://crrev.com/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/5f89d7ad5aa06e83421ac7d1c82ccab7e453e20e/tools/metrics/histograms/histograms.xml